## Introduction

Chikungunya virus (CHIKV) is a member of a group of globally distributed, mosquito-transmitted arthritogenic alphaviruses that cause sporadic outbreaks of primarily rheumatic disease every 2–50 years1,2,3,4. The largest epidemic of CHIKV disease (hereafter simply referred to as chikungunya) ever recorded began on Lamu Island, Kenya, in 2004 (Fig. 1). The epidemic expanded across four continents, with cases still being reported in 2019 (Fig. 1; Supplementary Table 1). Three major genotypes of CHIKV are now recognized — the Asian, the West African and the Asian and East–Central South African (ECSA) genotypes5 — but a new lineage, the Indian Ocean Lineage (IOL), also emerged from the ECSA genotype during the 2004–2019 epidemic6. The epidemic reached >100 countries (Supplementary Table 1), caused >10 million cases (Supplementary Table 2), and might arguably be called a pandemic. An estimated 1.3 billion people are at risk of chikungunya7. Climate change modelling suggests that many more areas of the world (including parts of China, sub-Saharan Africa, South America and the United States) might become able to accommodate transmission of CHIKV in the future8,9.

Chikungunya was previously often viewed as (and for many patients remains) a relatively benign and self-limiting rheumatic disease. However, a considerably more complex spectrum of less common atypical and severe manifestations is now recognized in subgroups of patients, with chikungunya often complicated by comorbidities and co-infections. Hospitalization rates for chikungunya range from 0.6% to 13%10,11,12,13,14 and estimates of chikungunya-related mortality range from 0.024% to 0.7%10,12,15,16,17. In addition, many patients develop protracted rheumatic disease lasting many months, occasionally years, with a number of sequelae now also recognized18,19,20,21,22. Estimates for the total economic costs (direct and indirect) of chikungunya have ranged from a median of US$67 for adults and$258 for children in Columbia23, to a mean of $150 per outpatient and$3,300 per inpatient in 2006 in Réunion Island11. In a large study of a chikungunya outbreak in Bangladesh in 2017, >10 days of productivity were lost in ~30% of patients with chikungunya because of severe arthropathy24. Such costs might be viewed as relatively modest by Western standards; however, the occasionally high attack rate of chikungunya, with up to 30–75% of a given population affected by chikungunya disease at any one time1,25, can result in a substantial economic burden, especially in resource-poor communities that are often affected by the disease26.

Considerable research in patients and animal models has now provided extensive insights into the complex spectrum of disease manifestations, the important antiviral factors and the central mediators of arthritic immunopathology. These insights have led to improved disease classification and management, and have spawned a plethora of potential avenues for new interventions. This Review provides an overview of the lessons learned about chikungunya in the aftermath of the 2004–2019 epidemic. The disease manifestations are outlined, including those associated with acute, atypical acute and severe acute disease, as well as the chronic phase of the disease and its potential sequelae. Disease in infants and children, and mother-to-child transmissions, are also discussed as the clinical presentations in this group of patients differ. Also covered are comorbidities, which increase the risk of severe disease, and co-infections with Zika virus (ZIKV) and dengue virus (DENV).

## Disease manifestations of chikungunya

Estimates for the asymptomatic infection rate for CHIKV range from 3% to 82%. The breadth of this range, derived from a comprehensive review of 24 studies, is similar to that found with other infectious diseases and has yet to be fully explained27. Four clinical forms of symptomatic chikungunya were proposed in an expert consultation, led by the WHO–Pan American Health Organization: acute, atypical acute, severe acute and chronic (suspected or confirmed)28,29. The three acute forms of chikungunya are associated with a range of different symptoms, with confirmation of diagnosis usually achieved by IgM serology (Box 1; Supplementary Table 3). Other classifications of chikungunya have included a sub-acute phase between acute and chronic30, with chronic disease defined as disease lasting >3 months30,31. Generally, patients with chronic disease do eventually recover (usually within 3–24 months)18,19, although sequelae might arise. An emerging body of evidence suggest that the IOL lineage is associated with more severe presentations than the Asian genotype19,32,33. Infection with other arthritogenic alphaviruses can cause similar acute symptoms (such as fever, polyarthralgia–polyarthritis, rash and myalgia)1, but rarely result in the atypical or severe manifestations that can occur with chikungunya, although such manifestations have been documented for Mayaro virus infections34.

### Acute chikungunya

The distinctive features of chikungunya onset are usually fever and polyarthralgia, often accompanied by polyarthritis (Table 1). The fever is often of rapid onset and high grade, with one large study reporting a mean maximum body temperature of 39.8 °C (SD ±0.5 °C) and fever duration of 4.88 days (SD ±2.7 days)24. Polyarthralgia usually starts around the same time as the fever and is often incapacitating, usually symmetrical and primarily involves peripheral joints19,24,35,36 (Fig. 2). Acute chikungunya commonly also involves a rash, which is usually maculopapular and predominantly located on the trunk and extremities, but also occurs less frequently on the face, palms, or soles19,37. The constellation of manifestations typically associated with acute disease (Table 1) seems to be considerably less common or less overt in older patients (>65 years of age), who have a much higher frequency of atypical or severe forms of chikungunya (as well as having a higher frequency of comorbidities) than younger patients29. Hence (along with inherent variability, different diagnostic criteria and different data acquisition processes), the wide ranges in the percentages of patients with certain symptoms (for example, 10–80% for myalgia) might also reflect the age distribution of patients included in the study cohorts.

### Atypical acute chikungunya

A large collection of atypical manifestations of acute chikungunya, affecting a range of systems and organs (for example, neurological, cardiovascular, skin, renal and respiratory manifestations) have been documented, often in hospital settings10,29,38,39,40 (Table 2). Although most patients with chikungunya admitted to hospital (~80% in one study10) do not have severe symptoms, many patients have atypical manifestations that can become severe and/or have chikungunya complicated by co-infections and/or comorbidities. Hospitalization rates for patients with chikungunya have varied from 0.6% (Martinique and Guadeloupe10), through 2.3% (Réunion Island11), 3.3% (Brazil12) and 6% (India13) to 13% (Puerto Rico14). The mean length of hospital stay reported for these patients was 5 days (SD ±7 days; range 0–146 days) in Réunion Island11 and 9 days (range 0–46 days) in Martinique and Guadeloupe10.

### Severe acute chikungunya

CHIKV infection can result in severe manifestations, the most prevalent being cardiac or multiple organ failure17,38,41 (Box 2). Chikungunya-associated viral sepsis and septic shock can also be fatal; for instance, in one study in Guadeloupe of patients with severe chikungunya, 25 out of 42 patients had septic shock, 12 of whom died42. Chikungunya can result in neurological complications43, with mortality often associated with central nervous system (CNS) diseases including encephalitis and encephalopathy44. Renal failure also seems to occur frequently in severe cases of chikungunya45 and is a reported cause of death38. Other rarer causes of chikungunya-related death that have been reported include toxic hepatitis, bullous dermatosis, myocarditis/pericarditis, respiratory failure, pneumonia and acute myocardial infarction38. Chikungunya-related mortality estimates vary for different regions: for example, studies have reported a mortality of 0.024% in Martinique and Guadeloupe10; 0.09% in Brazil17; 0.1% in Réunion Island16; 0.2% in India12; and 0.7% in the Dominican Republic15. However, the denominator for these percentages (that is, the total number of individuals with chikungunya in these regions) is often difficult to establish accurately in resource-poor settings. A mortality of 0.1%, derived from Réunion Island, might be viewed as a reliable estimate owing to the developed health care and reporting systems in this overseas region of France. Notably, old age (>40–75 years, depending on the study) is a risk factor for severe disease and mortality for individuals infected with either the Asian or IOL viruses15,17,30, whereas young age (<1 year) or old age (>65 years) increased the risk of CNS disease in a study on Réunion Island44.

### Chronic disease and sequelae

Arguably the most widespread cause of morbidity in patients with chikungunya is chronic disease, although the percentage, longevity, definition, terminology and evaluation of chronic disease vary widely between studies. A meta-analysis reported that ~25% of patients with chikungunya have had disease for >2 months and ~14% for >18 months18. Another meta-analysis of patients with chikungunya (that included patients with non-rheumatological manifestations) suggested that 43% of patients had not recovered within 3 months, and 21% had not recovered within 12 months19. However, a prospective study in India of 509 patients with chikungunya reported that all but 0.3% of the patients had recovered within 1 year46. The primary symptoms for chronic disease in patients with chikungunya are arthralgia and/or arthritis (up to 79% of patients with chronic disease), alopecia (10–29%) and depression (6–54%)20,22. Fatigue, mood disorders and sleep disorders were also common chronic symptoms19. Factors predisposing to chronic disease included comorbidities (such as osteoarthritis and diabetes), older age (>35–45 years for joint pain), and high viraemia and severe disease during the acute stage20. Chronic arthralgia in chikungunya generally involves the same joints affected during the acute phase (Fig. 2) and the arthropathy is not usually overtly erosive16,47.

Long-term sequelae of chikungunya include depression, chronic fatigue22 and other neurological disorders21. Difficulties remain in separating true sequelae from the progression of underlying comorbidities19,48, identifying the independent development of new disease entities and/or determining when patients with chikungunya have recovered and returned to the normal community background levels of musculoskeletal disease. The prevalence of musculoskeletal disease is increasing, with disability-adjusted life-years for musculoskeletal conditions having risen by 61.6% between 1990 and 2016, and by 19.6% between 2006 and 2016. Musculoskeletal conditions include >150 diagnoses, with about a third of people worldwide living with a chronic, painful musculoskeletal condition49. Therefore, a patient presenting with a musculoskeletal condition might be granted the same diagnostic rigour, regardless of whether or not they had a diagnosis of chikungunya >6–12 months previously.

### Disease in infants and children

Both infants and children can develop chikungunya after a mosquito bite, and neonates can be infected via mother-to-child transmission (Box 3). Infants (<1 year old) with chikungunya are often hospitalized and admitted to an intensive care unit (ICU)16,50. The disease usually presents as fever and rash51,52; arthralgia is difficult to assess in infants, but is perhaps expressed as irritability and excessive crying28,50. Skin rashes are common (~60–80%) and generalized, and include maculopapular rash, pigment changes, vesiculobullous lesions (fluid-filled lesions) and (sometimes extensive) desquamation (skin peeling)16,50,51,52. Atypical symptoms include (sometimes complex) seizures, diarrhoea, tachycardia, viral sepsis and septic shock51,52,53,54.

Acute chikungunya in children (from 1 to 18 years old) is comparable with disease in adults50,54,55, although the rate of asymptomatic infection might be higher overall than in adults27,56. Children (aged 2 months to 12 years) seem to generate stronger innate immune responses than adults55, which might explain the reduced severity of arthritis and lower rates of chronic arthropathy (5–11%) in children50,57. Nevertheless, a study from the 2014 epidemic in the Caribbean reported that 8.7% of children with chikungunya were hospitalized57. Common acute atypical manifestations include vomiting and seizures54. Severe disease primarily involves the CNS44,54,58, but chikungunya can also affect multiple systems and lead to severe viral sepsis and septic shock53,59. Severe manifestations are occasionally associated with mortality16,50,54.

## Comorbidities and co-infections

### Comorbidities

Comorbidities such as hypertension, diabetes (both type I and type II) and cardiac disease can contribute to chikungunya severity and admissions to the ICU60. For example, diabetes can increase the severity and duration of chikungunya and in patients with diabetes and hyperglycaemia, chikungunya infection is associated with worsening of diabetic symptoms (such as poor glycaemic control and acute complications)61. The presence of comorbidities is also associated with increased morbidity. For example, in a cross-sectional study of a chikungunya outbreak in north-eastern Brazil, 1% of patients with chikungunya had chronic kidney disease (amongst other comorbidities that included diabetes, haematological disorders, liver disease, hypertension and autoimmune diseases); these patients had higher frequencies of the main acute manifestations of chikungunya and higher mortality than patients without chronic kidney disease12. In another study of 65 patients with chikungunya who were admitted to ICUs in Martinique and Guadeloupe, 83% had pre-existing underlying comorbidities (hypertension, diabetes, renal disease, cardiac disease or autoimmune disease, including systemic lupus erythematosus) and the mortality rate among these patients was 27%62. Similarly, of 64 patients with chikungunya who were admitted to ICUs in French Polynesia, 77% had pre-existing conditions and 28% died63. The aforementioned comorbidities also often exacerbate disease after infection with other viruses such as DENV, West Nile virus and influenza virus60,61. Perhaps surprisingly, in a case series and literature review of patients with chikungunya undergoing a solid organ transplantation, most patients experienced no graft issues and a benign clinical course of chikungunya, with immunosuppressive treatment perhaps decreasing the risk of severe or chronic chikungunya immunopathological manifestations64.

### Co-infections

The symptoms, vectors and geographic distribution of the arboviruses DENV, ZIKV and CHIKV overlap considerably3,65,66. All three viruses are associated with fever, arthropathy and rash, which can complicate clinical diagnoses. All these viruses are also transmitted by Aedes aegypti and co-circulate in parts of South America, Africa and Asia, leading to co-infections. For instance, in a cohort of patients with febrile syndrome at the Colombian–Venezuelan border, 7.64% of patients were co-infected with both DENV and CHIKV, and 1.91% were co-infected with DENV, CHIKV and ZIKV67. Similarly, in a Nicaraguan study of patients with a suspected arboviral infection, 27% of patients tested positive for two or three of these viruses; however, the presence of DENV and/or ZIKV had no notable effects on CHIKV viraemia68. In a study in India, 12.4% of hospitalized patients with acute symptoms of chikungunya had IgM antibodies against both CHIKV and DENV; however, the only disease exacerbation associated with dual infection was diarrhoea (found in 16.2% of these patients)69. Patients with both chikungunya and dengue were also reported to have more severe arthropathy, myalgia, thrombocytopenia and rash than patients with dengue alone70. Such dual-infected patients were also more likely to have a rash and be hospitalized than patients with chikungunya alone71. The mortality rate is potentially higher in patients infected with both DENV and CHIKV than patients infected with either virus alone, although the evidence is weak given the very low patient numbers72,73. Finally, patients with chikungunya and a preceding DENV infection are at a higher risk of developing aggravated chronic chikungunya74. However, other studies have reported no notable exacerbation or unique presentations associated with acute dual or triple infections with the aforementioned arboviruses75,76. Thus, co-infections do not reliably cause novel clinical manifestations, nor do they generally seem to require unique clinical management77. However, patients with a potential DENV infection should not be given aspirin or other NSAIDs until they have been afebrile for ≥48 h and have no warning signs for severe dengue78.

Co-infections with CHIKV and either HIV79 or malaria have also been reported65. In patients infected with both HIV and CHIKV, lymphopenia was more common, more patients reached the definition of severe immunosuppression, and CD4 counts were lower than in patients infected with HIV alone79. Although the effect of CHIKV/malaria co-infections in humans remains unclear, mouse studies suggest that malaria infection can ameliorate chikungunya-related arthropathy80. In mice, CHIKV infection can compromise lymph node function81 and alter CD8 T cell trafficking82, with such CHIKV-mediated changes potentially modulating adaptive immunity and thus immunopathology in co-infection settings.

## Immunopathology

### Antiviral versus arthritic responses

In the advent of the recent unprecedented outbreak of chikungunya (Fig. 1), our understanding of the innate and adaptive immune responses induced by CHIKV infection, both in humans and in animal models, has grown substantially31,83,84,85,86,87. A range of cells and mediators have been implicated in chikungunya immunopathology (Supplementary Table 4). Importantly, many responses that promote chikungunya immunopathology are also required for protection against viral infections, which is clearly an important consideration in the development and application of new therapeutic interventions.

The type I interferons, primarily IFNβ and subtypes of IFNα, are antiviral cytokines that mediate highly effective protection against alphavirus infection88. These cytokines have an important function in limiting the sharp increase in viral replication during the early stages of infection88,89. The anti-alphaviral activity of type I interferons is optimal at 37 °C and this activity decreases with decreasing temperatures, being noticeably lower even with a reduction of only 2 °C (ref.36). In CHIKV-infected mice, the virus can replicate better in the extremities than elsewhere in the body because these tissues are usually a few degrees cooler, which might explain why the peripheral joints are usually affected in alphaviral arthropathies (Fig. 2)36. In the arthritic limbs of mice, up to ~8% of polyadenylated RNA can be of viral origin90, attesting to the extraordinary replicative capacity of CHIKV in the peripheral joints at slightly reduced temperatures. However, in addition to inhibiting viral replication, type I interferons can also promote arthritis. For example, injection of polyinosinic:polycytidylic acid (a mimic of viral double-strand RNA and potent inducer of type I interferon production) into the feet of mice can induce arthritis, and recapitulates much of the inflammatory gene expression signature that occurs in mouse feet during CHIKV arthropathy36.

In mice, B cells, T cells and natural killer (NK) cells are not required for survival during an acute infection91, whereas an intact type I IFN response is critical88,89. Deficiencies in components of the complex type I IFN network88 in the elderly (>65 years)92,93 and in neonates (<4 weeks old) might explain the increased risk of severe disease in these patient populations. For instance, neonates and very young children (<3 months) have attenuated RIG-I responses (required for the detection and triggering of type I IFN responses)94 and neonates have impaired interferon regulatory factor (IRF) 7 activation95 (required for amplification of the type I IFN response89). Monocytes from elderly individuals (>65 years old) have reduced expression of TNF receptor-associated factor 3 and IRF8 (both required for optimal RIG-I signalling) compared with monocytes from younger individuals92. Elderly individuals can also have slightly lower body temperatures than younger individuals96,97, which might also result in reduced antiviral type I interferon activity post-infection36.

The clearance of viraemia requires antiviral antibodies85,91,98. Neutralizing anti-CHIKV IgM responses are apparent as early as 4 days after the onset of symptoms99 and the presence of CHIKV-specific IgG3 antibody responses 7–10 days post-onset of symptoms is associated with more severe acute disease but decreased likelihood of persistent arthralgia100. CHIKV-specific CD4 T cells are required for IgG class switching and efficient production of anti-CHIKV IgG antibodies91, but these cells are also major promoters of arthritic inflammation.

Finally, monocytes and macrophages also have antiviral activity against CHIKV101,102,103, and are important for efferocytosis104 and resolution of inflammation105. However, as discussed in the next section, these cells are also highly implicated in chikungunya immunopathology.

A major objective for the field has been to identify appropriate pro-inflammatory mediators that can be targeted without compromising protective antiviral responses86,106. TNF is induced during CHIKV infections90,107,108,109 (Fig. 3), and TNF inhibitors (including etanercept and adalimumab) have shown some promise in the treatment of patients with chikungunya110. However, in mice with an active infection of Ross River virus (RRV) (a close relative of CHIKV that causes RRV disease), treatment with etanercept resulted in 100% mortality, indicating that TNF also has important antiviral activities111. This protective function of TNF against viral infections might raise concerns about TNF inhibitors for the treatment of patients with chikungunya; however, exacerbation or reactivation of CHIKV infection is unlikely once patients have adequate levels of neutralizing antibodies. Such antibodies (detectable 4 days after the onset of disease) are clearly present in patients with a positive serodiagnosis99 and have long been present by the time the chronic phase of disease begins112. Indeed, in patients with chronic manifestations of chikungunya, treatment with an immune-modulating biologic agent (including infliximab and etanercept) was not associated with overt worsening of disease113. An important issue to consider, however, is potentially compromising antiviral immunity in settings where multiple arboviruses are circulating. In such scenarios, therapies that target chikungunya immunopathology should ideally not compromise the patients’ ability to generate immunity to subsequent DENV or ZIKV infections.

In addition to the inhibition of antiviral activity, another potential concern of anti-inflammatory interventions is the risk of inadvertently promoting immunopathology. For instance, the chemokine CC-chemokine ligand 2 (CCL2) is strongly induced during CHIKV infections108,109 and targeting the CC-chemokine receptor 2 (CCR2)–CCL2 axis in mice reduces the recruitment of inflammatory monocytes and macrophages into the joints104. However, in the absence of monocytes and macrophages, neutrophils are instead recruited into the joints of CCR2–/– mice post-CHIKV infection, promoting joint destruction104.

### Mechanisms of immunopathology

Taking synovial biopsies or aspirates from patients with alphavirus-induced arthritis is often difficult to justify, as such procedures carry a small risk for the patient and usually have a negligible effect on disease management. Nevertheless, a small number of studies have analysed such material from patients with chikungunya or RRV disease112,114,115,116. As with other viral and bacterial arthritides117, CHIKV-related and RRV-related arthropathies probably arise from innate and adaptive immune responses stimulated by viral material in joint tissues1,90,91,109,118 (Fig. 3).

The ability of CHIKV to affect multiple systems/organs might be because of the virus’s predilection for infecting fibroblasts47,119, a cell type that is present in many tissues and organs (including the connective tissue, skin, synovium and periosteum89,119). The widespread expression of the arthritogenic alphavirus receptor, matrix remodelling-associated protein 8 (MXRA8), also probably contributes as this receptor permits infection of a large range of different cell types120. These cell types include circulating monocytes108, macrophages109, endothelial cells89,109, cells of the nervous system43,91 and skeletal muscle cells47,89, as well as cell types present in joints (Fig. 3). Infection usually induces cell death121, mainly by apoptosis,104,106 but also to a lesser extent by necroptosis and pyroptosis122. Cell death might directly contribute to pathology, especially for neurological manifestations91,122,123. However, immunopathology probably has the major role in the majority of rheumatic manifestations (Fig. 3, Supplementary Table 4).

#### Macrophages and monocytes

Arthritic infiltrates in patients with alphavirus-associated arthritides predominantly comprise mononuclear cells, mostly consisting of monocytes and macrophages but also including T cells, B cells and NK cells. In contrast to autoimmune arthritides, neutrophils are uncommon in the synovial infiltrates of patients with alphaviral arthritides101,112,114,115,116,124. Monocytes and macrophages are strongly implicated in chikungunya arthritic immunopathology

Synovial macrophages from patients with chikungunya have an activated morphology, with a ballooned appearance and multiple vacuoles112,116, indicative of a phagocytic (activated) phenotype108,125. Cytokines induced during CHIKV infection, such as type I interferons, IFNγ and TNF, are well-known activators of monocytes and macrophages (Fig. 3). Studies in non-human primates suggest that macrophages are the likely site of the persistence of CHIKV and CHIKV material109. Alphaviral RNA and/or proteins have also been detected in the synovial macrophages of patients with chikungunya and RRV disease112,114. In vitro work in RAW264 cells (a murine macrophage cell line) suggests that CHIKV-infected macrophages are a source of arthritogenic cytokines such as TNF and IL-6126. Mouse models of chikungunya arthritis and analysis of peripheral blood mononuclear cells from patients have also suggested a function for the NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome (and thus IL-1β and IL-18) in chikungunya, with a small-molecule inhibitor of NLRP3 activation able to reduce CHIKV-induced inflammation in mice127. Notably, NLRP3 is also implicated in the pathogenesis of rheumatoid arthritis (RA), with high levels of NLRP3 activation being reported in monocytes/macrophages infiltrating the synovia of patients with RA128.

As well as synovial macrophages, whole-blood RNA transcriptomic analyses in paediatric patients suggest that CHIKV infects peripheral blood monocytes (and dendritic cells) and that CHIKV induces a monocyte-centric pro-inflammatory response108. CCL2 is a major product of CHIKV-infected monocytes129, is strongly induced during CHIKV infection101,109 and is important for the recruitment of monocyte and macrophages into the inflamed joint104. CHIKV- infected monocytes also produce other pro-inflammatory mediators, including IFNα, IL-12 and CXC-chemokine ligand 10 (CXCL10)130 (Fig. 3).

Overall, monocytes and macrophages have a very large number of functions and differentiation states131 and interact with CHIKV on a range of levels; not only are these cells the sites of infection and persistence and the source of pro-inflammatory cytokines, these cells also have antiviral activity and are required for the resolution of inflammation.

#### CD4 T cells

CHIKV-specific CD4 T cells have been repeatedly implicated as important promoters of CHIKV-mediated arthritis80,132,133,134,135,136. Furthermore, regulatory T cells can ameliorate chikungunya arthropathy in mice137 and are also associated with disease resolution in humans134. In mouse models of chikungunya, CD4 T cells infiltrated into the joints in a CXC-chemokine receptor 3 (CXCR3)-dependent fashion80, with arthritogenic CD4 T cells seeming to have a dominant type 1 T helper (TH1) cell phenotype135, expressing the transcription factor T-bet104 and IFNγ32,90,101. Notably, IFNγ-expressing cells are also present in synovial biopsy samples from patients with RRV disease114. Curiously, IFNγ deficiency has no major effects on mouse models of chikungunya90,138. IFNγ expression was also not detectable in the synovial fluid of one patient with chronic chikungunya, although IFNγ was abundant in their blood during the acute disease phase112. The contribution of IFNγ in chikungunya thus remains unclear, although it should be noted that IFNγ is reported to have a complex and pleiotropic function in RA139,140.

More studies are required to better understand the mechanisms whereby CHIKV-specific CD4 T cells drive arthropathy. Conceivably, rather than being reliant on IFNγ, CHIKV-specific TH1 cells could activate monocytes and macrophages via interactions involving CD28 and CD80–CD86136,141, resulting in TNF and IL-6 production142. Alternatively, instead of IFNγ expression by TH1 cells, TNF expression by CD4 cells might have an important function in promoting chikungunya arthropathy135. TNF-expressing CD4 cells have a potential pathogenic function in psoriatic arthritis143 and are targeted by methotrexate in RA144,145; notably, methotrexate has shown some benefit in treating chikungunya146,147.

Some animal models104,109,148 have suggested the involvement of IL-17 and TH17 cells in alphaviral arthritides. Concentrations of IL-17 are marginally increased in the plasma of patients with chikungunya during the acute phase of disease compared with that of uninfected individuals149 and remain increased during the chronic phase150. By contrast, IL-17 is not increased in the blood of young patients with chikungunya108, who are known to experience less severe arthropathy than older patients. IL-17 is implicated in cartilage destruction and bone erosion in RA151, whereas radiographically detectable joint damage is not generally a feature of alphaviral arthropathy. Nevertheless, some patients with RRV disease have an increased receptor activator of nuclear factor kappa-B ligand (RANKL) to osteoprotegerin (OPG) ratio, indicative of increased osteoclastogenesis and bone resorption148. The levels of matrix metalloproteinase 2 (MMP2) messenger RNA were increased in the synovial fluid of one patient with chronic chikungunya (compared with levels in healthy individuals)112. Furthermore, concentrations of connective tissue metabolites (proline, hydroxyproline and mucopolysaccharides) were increased in the urine of patients with chikungunya during the first week post-onset of fever152. Thus, although alphaviral arthritides might be associated with some IL-17 production, cartilage destruction and bone erosion, the contribution of these processes to alphaviral rheumatic pathology seems to be substantially less important than their role in RA.

#### NK cells and NK T cells

Synovial NK cells (characterized by their CD56+ CD3 expression) in patients with chronic chikungunya express the activation marker CD69112, and data from mouse models suggest that NK cells have a pathogenic role in acute arthropathy32,90,91. NK cells from the peripheral blood of patients with acute chikungunya have an activated profile (including the expression of the heterodimer CD94:NKG2C) and are strongly cytotoxic, but secrete minimal levels of IFNγ153. NK cells in the peripheral blood of patients with chronic chikungunya express reduced levels of perforin, but they do not express notably higher levels of TNF or IFNγ compared with NK cells from healthy individuals107. Increased numbers of synovial CD56+ NK cells in patients with established RA probably promotes arthritis via secretion of TNF and IFNγ154; however, the mechanisms by which NK cells contribute to chikungunya arthropathy remain to be elucidated90.

In addition to NK cells, natural killer T (NKT) cells (characterized by their CD56+ and CD3+ expression) that express TNF or IFNγ are increased in the peripheral blood of patients with chronic chikungunya compared with healthy individuals, and are similarly increased in patients with RA compared with healthy individuals107. Hence, NKT cells are likely to contribute to both chikungunya and RA arthropathy, but the underlying mechanisms and their importance remain unclear.

#### Non-haematopoietic cells

Fibroblasts in connective tissues are a major target of CHIKV infection and produce various IFNα subtypes and IFNβ119, cytokines with well-described arthritogenic properties36. In vitro, CHIKV-infected human synovial fibroblasts secrete RANKL, IL-6, IL-8 and CCL2, and supernatants from these cultures can stimulate osteoclastogenesis155,156.

CHIKV can infect human osteoblasts in vitro, which promotes IL-6 and RANKL secretion by these cells and inhibits OPG secretion (increasing the RANKL to OPG ratio)157. Notably, the RANKL to OPG ratio is increased in the synovial fluid of patients with RRV disease compared with synovial fluid from healthy controls, and treatment with an anti-IL-6 antibody can reduce bone loss in a mouse model of RRV infection158. In addition to osteoblasts, CHIKV can also infect human chondrocytes in vitro120 and mouse chondrocytes in vivo89,159. RRV infection of chondrocytes induces the secretion of IL-6, CCL2, IFNγ and TNF160. Thus, multiple non-haematopoietic cell types in the joint can be infected with CHIKV and might contribute to the pro-inflammatory milieu (Fig. 3).

Human skeletal muscle cells (in vitro)120 and mouse skeletal muscle cells can also be infected by CHIKV in vivo89, although some evidence suggests that, in humans, only skeletal muscle progenitor (satellite) cells are infected161. Infection of skeletal muscle might be responsible for myalgia in patients with chikungunya (Table 1); notably, studies of mouse models of chikungunya have reported pronounced inflammatory infiltrates in skeletal muscle tissues101,111.

### Chronic arthropathy

The underlying inflammatory stimuli responsible for chronic chikungunya arthropathy remain unclear20,47. The persistence of the virus or viral material112,114,117, as well as host cell debris162, in joint tissues probably have important contributions. RNA-seq transcriptional profiling data from mice suggest that chronic inflammation in chikungunya is simply a prolongation of the acute inflammatory response90, which continues until the viral material is cleared91. The expression of a number of pro-inflammatory cytokines and chemokines in the peripheral blood are associated with chronic disease in patients with chikungunya46,83,118 (Supplementary Table 4). Notably, the expression of pro-inflammatory mediators IL-6, IL-8, CCL2 and IFNα were upregulated in the synovial fluid of a patient with chronic chikungunya compared with expression in serum from the same patient112.

In chikungunya, the virus and/or viral material seem primarily to persist in monocytes and macrophages in the joints109,112, which is a feature common to a number of arthritogenic viruses and bacteria in humans and animals117,163. Virus-induced apoptosis and reinfection of cells via apoptotic blebs is one postulated mechanism by which alphaviral infection might persist in vivo in the presence of neutralizing antibodies125,164. However, although viral material (RNA and/or protein) can be detected, researchers have been unable to isolate infectious (replication competent) virus from the joint tissues of patients with subacute or chronic disease112,114,116 or from mice 2 weeks after CHIKV infection91. A number of possibilities could explain this apparent discrepancy: levels of infectious virus might simply be too low to be isolated; defective viral RNA could continue to replicate as replicons without being able to produce infectious (replication-competent) virus165, with viral proteins being translated from replicon RNA, and/or residual inactive viral material is only slowly cleared91, with large amounts accumulating during the acute infection90.

Although CHIKV arthropathy shares many features with RA37,83,118,132, there is no clear evidence that autoimmune disease is involved or induced. However, viral material could not be found in synovial fluid from 38 patients with arthropathy at 22 months post-onset of chikungunya, perhaps arguing that arthropathy at this stage is no longer attributable to viral persistence and that alternative mechanisms are in play166. The investigators suggested that (as yet undefined) autoimmune sequelae might be responsible (although levels of cytokines or chemokines were not assessed in this study)166.

Arthralgia is a dominant feature of acute chikungunya and the main symptom of chronic chikungunya (Fig. 2). Alphaviral arthralgia probably involves inflammatory pain, perhaps driven by inflammatory cytokines; however, the mechanisms involved in both inflammatory pain in general167 and in alphaviral arthralgia specifically remain poorly understood. One might speculate that IL-6 has a role in driving alphaviral arthralgia, as this cytokine features prominently in acute and chronic chikungunya46,112 (Fig. 3) and anti-IL-6 drugs seem effective in treating pain and fatigue in RA168. Chikungunya arthralgia can also have neuropathic characteristics169,170, which might involve infection and disruption of cells of the peripheral nervous system21,43,91. Further research is needed to unravel the mechanisms that underpin alphaviral arthralgia, with such endeavours hopefully leading to new therapeutic approaches.

## Treatments and vaccines

### Anti-inflammatory therapy

A number of consensus guidelines30,146,171, reviews2,35,113,172,173,174 and perspectives37,175 for the treatment and management of chikungunya are available. In summary, acetaminophen (paracetamol) is recommended for the initial treatment of fever and pain. If pain-relief is inadequate, NSAIDs are the mainstay of treatment (except in patients with a suspected DENV infection78). However, NSAIDs are contraindicated in several comorbidities, including uncontrolled hypertension, kidney disease and inflammatory bowel disease, and NSAIDs should be discontinued in pregnant patients 6–8 weeks before birth. Low-dose corticosteroids (with or without NSAIDs) seem to be effective in NSAID-refractory patients35,113,146,171,175, although the potential adverse effects of these drugs should be considered in risk–benefit assessments31,176. For patients with chronic chikungunya who are refractory to the aforementioned treatments, DMARDs have shown some efficacy113,172, and sulfasalazine and methotrexate have been suggested as first-line options146,147. However, chikungunya arthropathy is usually not overtly erosive16,47, and so DMARDs might seem hard to justify177 unless an underlying destructive autoimmune disease is present. Methotrexate treatment, in particular, can have rare but potentially serious adverse effects and requires extensive clinical monitoring. Another DMARD, chloroquine, has also been reported to worsen disease178.

Targeting pathogenic CD4 T cells has shown some promise in animal models of chikungunya. For example, treatment with abatacept, a CTLA4-Ig fusion protein that interferes with T cell activation, ameliorated chikungunya in mice without affecting viraemia; however, this therapy was only partially effective unless combined with an antiviral antibody136. Although biologic drugs are an exciting new avenue for targeting specific arthritic pathways, the high cost of these drugs might preclude their widespread use, especially in resource-poor settings. Human data for the use of biologics in the treatment of chikungunya are also currently limited, inconclusive and/or complicated by autoimmune comorbidities113,172. Finally, fingolimod (a sphingosine 1-phosphate receptor agonist that is used to treat relapsing forms of multiple sclerosis179) has also shown preliminary potential for treating chikungunya. Treatment with this agonist, which sequesters lymphocytes in lymph nodes to prevent their participation in tissue inflammation, was able to abrogate chikungunya in a mouse model133; however, the cost of this drug might limit enthusiasm for this treatment in humans, especially in resource-poor settings.

### Antiviral treatments

There has been substantial preclinical evaluation of antiviral chemotherapeutic drugs for inhibiting CHIKV infection, which will not be reviewed herein as few of these drugs have been tested in vivo and none has reached or shown efficacy in human clinical trials. Whether an antiviral approach seeking to inhibit viral replication would be effective against CHIK is unclear. In most patients, by the time a diagnosis has been reached and treatment has been initiated, virus and/or viral RNA replication could be largely over. Conceivably, low-level RNA replication (evidence of which is currently lacking in patients) might drive chronic disease and could thus be targeted by antiviral drugs.

Antiviral monoclonal antibody treatments have also shown promise in mouse models103,180,181. However, by the time a serodiagnosis of chikungunya is obtained (Box 1), patients usually already have antiviral antibodies, and chronic disease occurs despite ongoing robust antibody responses112. The settings and window of opportunity wherein such antibody treatments might be effective might thus be quite limited133, and the high cost of such antibodies will probably limit their widespread use.

### Vaccine development

As well as treatment strategies, disease prevention measures (such as vaccines and mosquito control measures (Box 4)) are in development. CHIKV vaccines most often use the structural polyprotein of CHIKV; this polyprotein is cleaved into five proteins (E1 and E2 viral spike glycoproteins, capsid, E3 and 6 K) that assemble into a viral particle, thereby presenting an authentically folded quaternary structure to the immune system182,183. Vaccination seeks to recapitulate naturally acquired protective immunity (generated after infection with CHIKV) and induce neutralizing antibodies directed at the viral spike glycoproteins (comprising E1/E2 trimers). Such antibodies are thought to be the main mediators of protection98 by blocking the virus from binding to the receptor120, by blocking viral entry into cells and/or by preventing viral budding180,184.

The global market value for a CHIKV vaccine has been estimated to be ~€500 million (~US$600 million) annually; however, this value might be viewed as relatively low compared with other projects (for instance, the global influenza vaccine market value in 2018 was estimated to reach >US$ 5 billion)185. In a workshop in India in 2018, the Coalition for Epidemic Preparedness Innovations (CEPI) reported that four vaccines are in phase I human clinical trials and two vaccines are in phase II clinical trials186; the latter being a recombinant measles virus vaccine that encodes the CHIKV structural polyprotein187 and a virus-like-particle vaccine188. Unfortunately, phase III field trials for epidemic diseases such as CHIKV are complicated by the inability to predict the geographical location and size of the next outbreak189. An alternative or complementary approach to phase III field trials might involve a human challenge model (as described for dengue190), in which, for instance, vaccine recipients might be challenged with a live attenuated CHIKV191. Such studies might be combined with systems vaccinology and systems serology approaches, which should help to provide more sophisticated correlates of protection192,193.

All CHIKV genotypes seem to belong to a single serogroup194,195. However, variations in cross-neutralization capacities have been reported, such that antibodies raised to one CHIKV genotype are relatively less efficient at neutralizing a different CHIKV genotype33,196. Vaccines currently in development use CHIKV antigens from one CHIKV genotype; whether such (single valent) vaccines will provide broadly comparable protection against all CHIKV genotypes thus remains to be established. Another challenge will be timely deployment in rapidly evolving outbreaks; for example, during the Réunion Island outbreak, the number of infected individuals began to escalate at the beginning of 2006, but the epidemic was largely over by July 2006197.

## Conclusions

The unprecedented 2004–2019 CHIKV epidemic has resulted in a surge of research into chikungunya, and has led to many new insights and consensus guidelines for clinical management. There is a clear need for better treatment options for patients with chikungunya and NSAID-refractory arthropathy, chronic arthralgia or severe, life-threatening disease. Well underway are preclinical and clinical investigations of new drugs, and drugs developed for other inflammatory arthritides (such as RA), for treating chikungunya arthropathy. Such endeavours should also facilitate treatment of arthritic disease caused by other alphaviruses such as Mayaro virus and RRV, which have the potential to cause alphavirus outbreaks34,198. However, expensive treatments are unlikely to be widely adopted in resource-poor communities and in high-attack-rate settings. Protracted chronic chikungunya (particularly chronic arthralgia) remains poorly understood and complicated by comorbidities and high background levels of musculoskeletal pain in the community. Mosquito control measures are hampered by insecticide resistance and the difficulties in judging whether interventions actually effect disease prevalence. Some vaccines are in advanced stages of development; however, the limited market size does not provide a clear financial incentive for the development of vaccines, and outbreaks are unpredictable. New agencies (such as the CEPI) and technologies are probably needed to bring such interventions to the market193.